Utility lines for water, electricity, gas, telephone and cable television are often run underground for reasons of safety and aesthetics. Sometimes, the underground utilities are buried in a trench that is then back filled. However, trenching can be time consuming and can cause substantial damage to existing structures or roadways. Consequently, alternative techniques such as horizontal directional drilling (“HDD”) are becoming increasingly popular.
A typical horizontal directional drilling machine includes a frame on which is mounted a rotational drive mechanism that can be slidably moved along the longitudinal axis of the frame, to rotate a drill string about its longitudinal axis while sliding along the frame to advance the drill sting into, or withdraw it from, the ground. The drill string comprises one or more drill rods attached together in a string.
A boring tool is installed onto the end of the drill string furthest away from the horizontal HDD machine. For example, a drill bit is used when the drill string is being advanced into the ground where there is no existing hole. Similarly, a back reamer is used to enlarge a bored hole and is used when the drill string is being withdrawn after a hole is cut. These boring tools may include a wide variety of soil cutting devices tailored for specific formations. Examples include cutting edges that shear the soil and compression elements that concentrate longitudinal force from the drill string onto a concentrated area to fracture the ground when boring in rock conditions. In either case, the operation of the boring tools includes both rotational and longitudinal (or thrust) motion.
Boring machines include controls that allow the operator to control both the rotational movement and the longitudinal movement, also referred to as thrust, of the drill string and consequently of the boring tool. Typically, the magnitude of the rotational movement and thrust movement are proportional to the position of the controls. The optimum setting of rotational movement and thrust movement depends on various factors such as the soil conditions, the formation, and the type of boring tool. It is therefore necessary for the operator to establish the optimum setting based on each unique boring situation. However, in some situations the soil conditions can change rapidly, particularly as the boring tool advances through the soil and encounters soils of different densities and types, such as clay soil and rocks. Under these circumstances, an operator may be not be able to adjust the settings quickly enough to compensate for these variations. U.S. Pat. No. 5,746,278, to Bischel, herein incorporated by reference, discloses a control system that automatically adjusts the rotational movement and thrust movement settings, independently from the inputs of the operator.
In some conditions, the boring process requires maintaining consistent values of the rotational and thrust movement settings, which in turn requires the operator to maintain the controls in the appropriate position for relatively long periods of time. It can be difficult, however, for the operator to accurately maintain the positions of the controls for relatively long periods of time without becoming fatigued or losing attentiveness. In these conditions, the control system can be set to automatically maintain the boring parameters once the operator has determined the optimum levels of rotation and thrust. A control system configured in this way allows the operator to first manually set the desired rotational movement and thrust movement parameters, and then to maintain this state by depressing a separate control (such as a switch) that causes the control system to maintain these settings when the operator lets go of the controls. Although the controls typically return to their neutral positions (the position where the rotational and thrust movement are set to zero), the rotation and thrust movement settings are maintained automatically at the preferred operating state.
The boring operation must generally, however, be periodically interrupted, such as when a drill rod needs to be added to the drill string during boring or when a drill rod needs to be removed from the drill string during backreaming. When the boring process is resumed, the drill bit must be transitioned from a stationary state to a drilling state. A drilling state may generally be defined as including rotation and thrust against the soil. To accomplish this, the control system may further be configured to resume the rotational movement and thrust movement parameters that were present before the boring operation was interrupted. However, when the control system attempts to quickly resume the rotational and thrust movement settings, high loads can be encountered in the boring tool and drill string. These high loads can damage the boring tool and drill string and lead to poor drilling performance. Therefore, there is a need for an optimized boring resumption process and an apparatus for implementing the same.